1. Field of the Invention
The invention relates to a road lane marker detection apparatus and a road lane marker detection method. More particularly, the invention relates to a road lane marker detection apparatus and road lane marker detection method capable of more reliably detecting a line lane marker and a dotted lane marker even if one of the road lane markers, from among a road lane marker on the left side of the vehicle and a road lane marker on the right side of the vehicle, is a line lane marker such as a white line and the other road lane marker is a dotted lane marker formed by Botts' dots or the like.
2. Description of the Related Art
In recent years, drive assist systems such as a lane keeping assist system (LKA) have been developed to reduce the load on the driver of a vehicle. The lane keeping assist system is provided with an imaging portion that captures an image of a road lane marker such as a white line using an on-board camera, a feature point obtaining portion that obtains feature points of the road lane markers based on the obtained captured image, a lane marker detecting portion that detects a road lane marker based on the obtained feature points, and a handling assist portion that assists with handling based on the positional relationship between the detected road lane marker and the host vehicle.
A road lane marker is the line that demarcates a running lane. One running lane is demarcated by two road lane markers, one on the right side of the vehicle and one on the left side of the vehicle. The road lane marker is typically a line lane marker such as a white line or a yellow line. However, in the United States, there are also dotted lane markers that are formed by a series of round raised road markers referred to as Botts' dots. Moreover, in Japan and the United States there are dotted lane markers that are formed by a series of round raised road markers referred to as cat's eyes. Hereinafter, white lines and yellow lines will hereinafter collectively be referred to as “white line or the like,” and Botts' dots and cat's eyes will collectively be referred to as “Botts' dots or the like.”
Compared to a white line or the like, Botts' dots or the like have low contrast with asphalt and have poor continuity in the length direction of the road. Consequently, they are not as visible as a white line. Therefore, in order to detect a road lane marker based on a captured image, it is preferable to use a different method to analyze an image for a white line or the like than is used to analyze an image for Botts' dots or the like. This is because the detection accuracy may decrease if an attempt is made to uniformly detect both a white line or the like and Botts' dots or the like using the same method of analysis.
First, an example of a method for detecting a white line or the like will be described. The feature points of a line lane marker such as a white line or the like are the points that form the outline of the line lane marker. In the lane keeping assist system, the feature point obtaining portion obtains the feature points by detecting the contrast in the luminance at the boundary between the asphalt and the line lane marker. The feature point obtaining portion searches for the feature points in each row of pixels in the width direction of the road in a predetermined region that includes the road lane marker on the left side of the vehicle and the road lane marker on the right side of the vehicle. The feature point obtaining portion searches the rows of pixels discontinuously (discretely) in the length direction of the road. Accordingly, a low density search for the feature points is conducted in the predetermined region. The reason for this is that the continuity of a white line or the like is high so many feature points can be obtained, and thus the accuracy with which a white line or the like can be detected is high, even if the feature points are detected with a low density search. The lane marker detecting portion obtains a line that resembles a row of feature points by performing a calculation such as a Hough conversion on the obtained feature points. As a result, the lane marker detecting portion detects a line lane marker.
Next, an example of a method for detecting Botts' dots or the like will be described. The feature points of a dotted lane marker formed by Botts' dots or the like are the points that form the outline of the Botts' dots or the like. In the lane keeping assist system, the feature point obtaining portion obtains the feature points by performing a morphological operation on the captured image. The feature point obtaining portion searches for the feature points in each row of pixels in the width direction in a predetermined region that includes the road lane marker on the left side of the vehicle and the road lane marker on the right side of the vehicle. The feature point obtaining portion searches the rows of pixels continuously in the length direction of the road. Accordingly, a high density search is conducted for the feature points in the predetermined region. The reason for this is that the continuity of Botts' dots or the like is poor so a sufficient number of feature points cannot be obtained, and thus Botts' dots or the like cannot be reliably detected, unless a high density search for the feature points is conducted. The feature point obtaining portion detects a dotted lane marker based on the obtained feature points.
When both the road lane marker on the left side of the vehicle and the road lane marker on the right side of the vehicle are white lines or the like, the road lane markers may be detected using the method for detecting a white line or the like described above. Also, when both the road lane marker on the left side of the vehicle and the road lane marker on the right side of the vehicle are formed by Botts' dots or the like, the road lane markers may be detected using the method for detecting Botts' dots or the like described above.
Botts' dots or the like are provided in a location between lanes on a road with a plurality of lanes in one direction, but are not provided on either edge of the road, i.e., in a location that separates a running lane from the shoulder of the road (i.e., along the edge of the road). A white line or the like is provided on both edges of the road. Therefore, the road lane marker on one side of both the left and right lanes is a white line or the like and the road lane marker on the other side of both the left and right lanes is formed by Botts' dots or the like. Typically in this case, the white line or the like is detected by the method for detecting a white line or the like described above and the Botts' dots or the like are detected by the method for detecting Botts' dots or the like described above.
In this case, the captured image received by the feature point obtaining portion includes both the white lines or the like and the Botts' dots or the like. As described above, with the method for detecting the Botts' dots or the like, a high density search for the feature points is conducted in a predetermined region of the captured image. Considering the processing speed of the CPU, an upper limit value is set for the number of feature points obtained per one frame of the image, and this upper limit value is set at a total value for the combined number of feature points of the road lane marker on the left side of the vehicle and the road lane marker on the right side of the lane marker. Therefore, if an extremely large number of feature points are obtained at a white line or the like, which has high continuity, and the number of feature points ends up exceeding the upper limit value before the number of feature points necessary to detect a dotted lane marker at Botts' dots or the like, which have poor continuity, is obtained, the feature point obtaining process will end up stopping at that point. If this happens, a dotted lane marker that is formed by Botts' dots or the like may not be able to be detected.
Japanese Patent Application Publication No. 2005-182303 (JP-A-2005-182303) describes an invention related to a Botts' dots detection apparatus. However, with this apparatus as well, the problem described above still arises when one road lane marker is a white line or the like and the other road lane marker is formed by Botts' dots.